Brian Dunbar
Headquarters, Washington, D.C.
July 21, 1994
(Phone: 202/358-1547)
Diane Ainsworth
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone: 818/354-5011)
RELEASE: 94-122
APOLLO REFLECTORS CONTINUE TO AID STUDIES OF THE MOON
A quarter of a century ago, Apollo 11 astronauts Neil
Armstrong and Buzz Aldrin deployed a variety of scientific
experiments in the fine powder of the Sea of Tranquility.
Among those devices was a laser ranging retroreflector,
which, a generation later, is still yielding fundamental
scientific data.
Scientists who analyze data from the Lunar Laser Ranging
Experiment have reported some watershed results from these
long-term experiments, said team investigator Dr. Jean Dickey at
NASA's Jet Propulsion Laboratory, Pasadena, Calif. The team's
findings appear in this week's issue of Science, which
commemorates the silver anniversary of the Apollo 11 lunar
landing.
Laser ranging has made possible a wealth of new information
about the dynamics and structure of the Moon. Among many new
observations, scientists now believe that the Moon may harbor a
liquid core. The theory has been proposed from data on the
Moon's rate of rotation and very slight bobbing motions caused by
gravitational forces from the Sun and Earth.
Other recent findings from the laser ranging experiments
include:
Verification of Einstein's theory of relativity, which
states that all bodies fall with the same acceleration
regardless of their mass.
The length of an Earth day has distinct small-scale
variations, changing by about one thousandth of a second
over the course of a year. These changes are caused by the
atmosphere, tides and the Earth's core.
Precise positions of the laser ranging observatories on
Earth are slowly drifting as the crustal plates on Earth
drift. An observatory in Hawaii is seen to be drifting away
from an observatory in Texas.
Ocean tides on Earth have a direct influence on the Moon's
orbit. Measurements show that the Moon is receding from
Earth at a rate of about 1.5 inches (about 3.8 centimeters)
per year.
Lunar ranging has greatly improved scientists' knowledge of
the Moon's orbit, enough to permit accurate analyses of
solar eclipses as far back as 1400 B.C.
Continued improvements in range determinations and the need
for monitoring the details of the Earth's rotation will keep the
lunar reflector experiments in service for years to come, Dickey
said.
"Using the Lunar Laser Ranging Experiment, we have been able
to improve, by orders of magnitude, measurements of the Moon's
rotation," Dickey said. "We also have strong evidence that the
Moon has a liquid core, and laser ranging has allowed us to
determine with great accuracy the rate at which the Moon is
gradually receding from the Earth."
The first laser ranging retroreflector was positioned on the
Moon in 1969 by the Apollo 11 astronauts so that it would point
toward Earth and be able to reflect pulses of laser light fired
from the ground.
By beaming laser pulses at the reflector, scientists have
been able to determine the round-trip travel time of a laser
pulse and provide the distance between these two bodies at any
given time down to an accuracy of about 1 inch (about 2.5
centimeters).
The laser reflector consists of 100 fused silica half-cubes,
called corner cubes, mounted in an 18-inch-square (46-centimeter)
aluminum panel. Each corner cube is 1.5 inches (3.8 centimeters)
in diameter. Corner cubes reflect a beam of light directly back
toward the point of origin, allowing scientists to measure the
Earth-Moon separation and study the dynamics of the Earth, the
Moon and the Earth-Moon system.
Once the laser ranging experiments began to yield valuable
results, more reflectors were left on the Moon. A reflector
identical to the Apollo 11 mission reflector was left by the
Apollo 14 crew, and a larger reflector using 300 corner cubes was
placed on the Moon by the Apollo 15 astronauts. French-built
reflectors were also left on the Moon by the unmanned Russian
Lunakhod 2 mission.
Several observatories have regularly ranged the Moon with
these reflectors: one is located at McDonald Observatory near
Fort Davis, Texas; another is located atop the extinct Haleakala
volcano on the island of Maui in Hawaii; another is located in
southern France near Grasse.
The Lick Observatory in northern California also has been
used in the past for the lunar laser ranging experiments, and
ranging programs have been carried out in Australia, Russia and
Germany. Despite the difficulty of detecting reflected laser
light from the Moon, Dickey said, more than 8,300 ranges have
been measured over the last 25 years.
"Lunar ranging involves sending a laser beam through an
optical telescope," Dickey said. "The beam enters the telescope
where the eye piece would be, and the transmitted beam is
expanded to become the diameter of the main mirror, then bounced
off the surface toward the reflector on the Moon."
The reflectors are too small to be seen from Earth, so even
when the beam is precisely aligned in the telescope, actually
hitting a lunar retroreflector array is technically challenging.
At the Moon's surface the beam is roughly four miles wide.
Scientists liken the task of aiming the beam to using a rifle to
hit a moving dime two miles away.
Once the laser beam hits a reflector, scientists at the
ranging observatories use extremely sensitive filtering and
amplification equipment to detect the return signal, which is far
too weak to be seen with the human eye. Even under good
atmospheric viewing conditions, only one photon -- the
fundamental particle of light -- will be received every few
seconds.
The range accuracy of these reflectors has been improved
over the lifetime of the lunar laser ranging experiments, the
team noted in Science. While the earliest ranges had accuracies
of several yards (or meters), continuing improvements in the
lasers and the detection electronics have led to recent
measurements that are accurate to about 1 inch (2.5 centimeters).
From the ranging experiments, scientists know that the
average distance between the centers of the Earth and the Moon is
239,000 miles (382,000 kilometers), showing that modern lunar
ranges have relative accuracies of better than one part in ten
billion.
"This level of accuracy represents one of the most precise
distance measurements ever made," Dickey said. "The degree of
accuracy is equivalent to determining the distance between Los
Angeles and New York to one fiftieth of an inch.
At JPL the lunar ranging analysis is carried out by JPL
scientists Drs. Jean Dickey, James G. Williams, X X Newhall and
Charles F. Yoder. The work is sponsored jointly by the
Astrophysics Division of NASA's Office of Space Science and the
Solid Earth Science Branch of NASA's Mission to Planet Earth
Office, Washington, D.C. Additional work is done at the Joint
Institute for Laboratory Astrophysics at the University of
Colorado at Boulder; at the University of Texas at Austin; and at
Observatoire de la Cote d'Azur, Grasse, France.
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